For all that we have instant-runoff preferential voting, we still have single-member districts for our House of Representatives (and 150 members of said House). We also have compulsory voting and (at the federal level, and in most states), a full-preferential ballot.

When you have 150 single-member districts you're probably going to get two major parties regardless of whether you have FPTP or IRV or even Approval voting. What compulsory voting and preferencing add on top of that is both of Australia's majors have to win the centre, rather than energising their base per the US.

What our system does allow for is for smaller parties in our Senate. It's generally elected 6 people at a time, allowing for e.g. 1 Green, 2 Labor, 2 Liberal-National and 1 One Nation (this is a very plausible result for Queensland next year).

So what does the US system allow then? Well, every system has to let its steam out somewhere. The US has its primaries, which often has a level of access and openness that greatly surpasses any Australian preselection (generally still getting out of the smoke-filled-room era).